Semiconductor lasers are widely used as a light source for ultra-high speed and long distance optical fiber communication. However, due to a little reflecting-back light from an optical fiber, there occurs a noise in conventional semiconductor lasers. Because of this, an optical isolator needs to be always used as an optics component for removing the reflecting-back light to the semiconductor laser.
In general, conventional optical isolators comprise a magnetooptic crystal with Faraday effect and two polarizers which are disposed sandwiching the magnetooptic crystal and have polarization planes shifted 45.degree. to each other. In the conventional optical isolators, light emitted from a semiconductor laser is transmitted through the first polarizer, entered into the magnetooptic crystal, where the polarization plane is rotated 45.degree. in a predetermined direction, then transmitted through the second polarizer without being attenuated. On the other hand, reflecting-back light is transmitted through the second polarizer, entered into the magnetooptic crystal, where the polarization plane is further rotated 45.degree. in the predetermined direction. Thereby, the polarization plane of the reflecting-back light is shifted by 90.degree. to the polarization plane of the first polarizer, therefore the first polarizer can prevent the reflecting-back light from entering into the semiconductor laser.
However, in the conventional optical isolators, it is required to prepare at least four elements including a magnet and fabricate them, therefore they are very costly.
Moreover, it is, in fact, difficult for the conventional optical isolator to be monolithically integrated with the semiconductor laser.